Over Shabbat, I had a guest that do not keep kosher and I can say with certainty they recently ate non-kosher food. Are there any kashrut concerns with the glass cup and cutlery they used at my house? I think it is sufficient to wash them in cold water with soap but wanted to check.
There is no need for you to be concerned with this. Even if they did eat non-kosher food trief, the amount of it will be botil and insignificant. You are right, it is sufficient to wash them out with cold water and soap.